This application is based on Japanese Patent Application No. 9-127398, filed on May 16, 1997, and Japanese Patent Application No. 9-127399, filed on May 16, 1998, the content of which are cited herein by reference.
The present invention relates to a computer system and a video decoder, and more particularly to a computer system having a function of decoding digital compressed codes of motion-picture data and displaying the motion-picture data on a display monitor of a noninterlace display system, and a video decoder used in the system.
With the recent development in the field of computers and multimedia technology, more and more computers capable of executing multimedia applications are developed. The computers of this type have a function of reproducing not only text and graphics data but also motion pictures and sound data.
In accordance with an increase in the number of multimedia computers, DVDs are attracting the attention of those skilled in the art, as a new type of storage media replacing CD-ROMs. About 4.7 GB of data, which is about seven times greater in amount than the data recorded on a traditional CD-ROM, can be recorded on one side of a DVD-ROM, and about 9.4 GB of data can be recorded on two sides of the DVD-ROM. The use of the DVD-ROM enables movie titles, which contain a large amount of video information, to be reproduced on a computer screen with high quality.
Video information stored on a DVD-ROM is made up of two kinds of data: presentation data and navigation data. The presentation data is data on video objects to be reproduced, and includes video, sub-picture and audio data. The video data is 1 and compressed according to MPEG-2, while the sub-picture and the audio data are compressed according to run-length encoding or Dolby digital system. The sub-picture data is bit-map data, and used to represent movie titles or display a number of choice items on a menu screen. One video object can include video data of one channel, audio data of up to 8 channels, and sub-picture data of up to 32 channels.
The navigation data is reproduction control data used for controlling the order in which presentation data are reproduced, and navigation commands can be embedded in the navigation data. The navigation commands are used for changing the contents of video data to be reproduced and the order in which they are reproduced. By using the navigation commands, the person who prepares the movie titles can define various tree structures in them, so as to make the movie titles interactive.
The titles are prepared on the assumption that they will be reproduced on a home TV by use of a commercially-available player. The contents of the titles are the following two:
(1) frame data (progressive data) which is encoded at a frame rate of 24 Hz, like a movie film; and
(2) field data which is encoded at a rate of 60 Hz (i.e., the number of field data pieces used per second is 60).
Some of the titles are a combination of data (1) and (2) above. For example, a program is constituted by data (1), namely, frame data (progressive data), while CM information is constituted by field data.
Both the frame data (progressive data) and the field data (data [1] and data [2]) are decoded by a DVD decoder. This DVD decoder outputs 60 Hz field data for interlace display corresponding to an NTSC output. In order to display the field data on the display monitor of a computer, the data for interlace display output from the DVD decoder has to be converted into data for noninterlace display. This conversion is executed by use of simple field synthesis. That is, an even-numbered field and an odd-numbered field are synthesized in a frame memory to produce one frame, and this frame is shown on the display monitor of a computer in a noninterlace fashion.
According to the interlace/noninterlace conversion based on the single field synthesis, fields between which a time difference exists are synthesized into one frame. This results in a so-called xe2x80x9cfeatheringxe2x80x9d phenomenon, wherein the outline of the frame appears to. have a fringe. This phenomenon is marked particularly in the case of a quick motion scene.
The xe2x80x9cfeatheringxe2x80x9d phenomenon is attributable to the following two causes:
(i) Field Picture Problem
This problem is due to the synthesis of field data (data [2]). Since each field picture corresponds to a time difference of 1/60 seconds, synthesis of them inevitably produces xe2x80x9cfeatheringxe2x80x9d.
(ii) Repeat Field Problem:
When a DVD decoder decodes frame data (progressive data) of 24 frames/sec, it uses a method called xe2x80x9c3:2 pulldownxe2x80x9d. By this method, frame data of 24 frames/sec is converted into field data of 60 fields/sec. The xe2x80x9c3:2 pulldownxe2x80x9d method is shown in FIG. 11.
Referring to FIG. 11, xe2x80x9cFrame No.xe2x80x9d indicates the frame number of frame data (24 frames/sec) which is not yet decoded. xe2x80x9cField No.xe2x80x9d indicates the field number of field data (60 fields/sec) which is already decoded and corresponds to NTSC. The suffix letter xe2x80x9cExe2x80x9d of the field number indicates that the field is even-numbered, while the suffix letter xe2x80x9cOxe2x80x9d indicates that the field is odd-numbered.
In the xe2x80x9c3:2 pulldownxe2x80x9d method, the difference in frame rate is dealt with by preparing three fields from one frame, and two of three frames are used in the processing for preparing three fields. When three fields are prepared, the third field is the same data as the first field (R: a repeat field). In FIG. 11, the third field of the first frame is a repetition of the first field (1E), and the third field of the third frame is also a repetition of the first field (3O).
In the simple field synthesis processing, consecutive two field pictures (namely, 1E and 1O, 1E and 2O, 2E and 3O, . . . ) are synthesized together, as shown in FIG. 11. What becomes a problem at the time of synthesis is the combination between fields 1E and 2O and the combination between fields 2E and 3O. Fields 1E and 2O are prepared from data of different frame numbers, and there is a time difference of 1/24 seconds between fields 1E and 2O. Likewise, there is a time difference of 1/24 seconds between fields 2E and 3O. As can be seen from this, a frame produced by a combination of fields that includes a repeat field causes xe2x80x9cfeathering.xe2x80x9d
As described above, if titles prepared for a TV are displayed on a display monitor of noninterlace display system, the field picture problem and repeat field problem described above occur, resulting in xe2x80x9cfeatheringxe2x80x9d. In particular, where titles are prepared from frame data that must be processed in the xe2x80x9c3:2 pulldownxe2x80x9d method, half of the field combinations includes fields between which a time difference exists. In comparison with the quick motion picture shown on a TV, the quick motion picture shown on a display monitor of noninterlace display system is poor in quality.
Accordingly, an object of the present invention is to provide a computer system and a video decoder that enable a xe2x80x9cfeatheringxe2x80x9dxe2x80x94free smooth image to be displayed on the display monitor of a computer by preventing fields between which a time difference exists from being synthesized or combined.
Another object of the present invention is to provide a computer system and a video decoder that enable a xe2x80x9cfeatheringxe2x80x9dxe2x80x94free smooth image to be displayed on the display monitor of a computer by means of a simple hardware structure capable of excluding repeat fields from the fields to be combined.
To achieve these objects, the present invention provides a computer system which is provided with a decoder for decoding a video data stream that is in the form of digital compressed codes, which converts video data output from the decoder and adapted for interlace display into video data suitable for noninterlace display, and which displays the resultant video data on a display monitor, the computer system comprising: conversion means for converting the video data adapted for interlace display into the video data suitable for noninterlace display in one of two conversion modes including a field synthesis mode in which a frame for noninterlace display is generated by synthesizing an odd-numbered field and an even-numbered field together, and an interpolation mode in which a frame for noninterlace display is generated by interpolating odd-numbered or even-numbered lines in fields; and switching means for determining whether video data obtained by the decoding by the decoder is field data, or frame data that has to be converted into a plurality of field data in units of one frame, and switching the conversion modes of the conversion means from one to another on the basis of results of determination.
The computer system of the present invention selects a conversion mode in which to convert data from interlace format data to noninterlace format data, by checking the structure of the video data obtained by the decoding by the decoder. The interpolation mode is selected when the video data is field data, and the field synthesis mode is selected when it is frame data. In this manner, the conversion modes can be dynamically switched from one to the other.
In the interpolation mode, lines are interpolated in the fields output from the decoder (an odd-numbered line is added to an even-numbered field, and an even-numbered line is added to an odd-numbered field), and one frame picture is generated from one field picture. Therefore, synthesis of the fields between which a time difference exists can be prevented. The frame data is processed such that data having the same frame number is divided into a plurality of fields. Therefore, with respect to the frames other than those frames from which repeat fields are produced, field data of the same frame number are synthesized in the field synthesis mode, thus preventing synthesis of fields between which a time difference exists.
When the video data decoded by the video decoder is a repeat field produced in the 3:2 pulldown conversion, it is excluded from the data to be subjected to the conversion processing which is executed by the conversion means in the field synthesis mode. Since the repeat fields are skipped when the field synthesis is executed, the repeat field of one frame is not synthesized with the first field of the next frame.
In general, the conversion means is provided as a function of the display controller. In this case, the conversion mode is switched from one to the other by either hardware or software. According to the hardware technique, a switching signal from the video decoder is supplied directly to the display controller. According to the software technique, the CPU receives an interruption signal indicative of the structure of the video data to be supplied to the decoder, and the conversion mode of the display controller is controlled based on software.
The skipping of repeat fields is controlled by either hardware or software. That is, a signal indicative of the generation of a repeat field is supplied from the video decoder to the display controller, and this can be realized based on either a hardware-based or software-based technique.
Where the video decoder has a function of making notification of the structure of video data or the generation of a repeat field, as mentioned above, the conversion function of the display controller (i.e., the conversion from the interlace format to the noninterlace format of display data) can be controlled with high efficiency. Hence, a xe2x80x9cfeatheringxe2x80x9dxe2x80x94free smooth image can be shown on the display monitor of a computer.
The present invention also provides a computer system comprising a video decoder for decoding a video data stream that is in the form of digital compressed codes, and a display controller, having a video port for receiving video data for interlace display output from the decoder, for converting the video data received at the video port into video data for noninterlace display in a field synthesis mode in which an odd-numbered field and an even-numbered field are synthesized together to produce a frame for noninterlace display, and for displaying the video data for noninterlace display, the computer system further comprising means for masking a vertical synchronizing signal and a horizontal synchronizing signal received at the video port of the display controller when the video data output from the video decoder is a repeat field produced by 3:2 pulldown conversion.
In this computer system, frame data input to the video decoder is checked to see whether it requires 3:2 pulldown conversion. When the frame data requiring 3:2 pulldown conversion is input, the video decoder executes a decoding operation that involves 3:2 pulldown conversion, and outputs a repeat field periodically. This repeat field is supplied to the video input port of the display controller, and the vertical and horizontal synchronizing signals that are output to the video input port at the time, are masked. Since the vertical and horizontal synchronizing signals are basically used for capturing video data, repeat fields are prevented from being supplied to the video input port by masking the vertical and horizontal synchronizing signals. The repeat fields are thus excluded from the fields to be combined, the simple field synthesis processing is executed by combinations between the two fields of the same frame number. In this manner, a xe2x80x9cfeatheringxe2x80x9dxe2x80x94free smooth image can be reproduced on the display monitor of a computer by merely adding a simple hardware component that masks the vertical and horizontal scanning signals.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinbefore.